Semiconductor Memory devices can generally be categorized as either volatile or non-volatile, depending on their speed and data retention characteristics. Typical volatile memory devices include random access memory (RAM). RAM devices are typically volatile in that stored data is lost once the power source is disconnected or removed. Non-limiting examples of RAM devices include dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM) and static random access memory (SRAM). In addition, DRAMs and SDRAMs also typically store data in capacitors which require periodic refreshing to maintain the stored data.
Non-volatile memory devices, such as erasable programmable read only memory (“EPROM”), retain data almost indefinitely (more than ten years) without an external power supply. Usually, bytes or words can be erased and reprogrammed individually during system operation. EPROM is more expensive and less dense than RAM and is appropriate for storing small amounts of data which is changed infrequently.
In recent years, the number and density of memory elements in memory devices has been increasing. Accordingly, the size of each element has been shrinking, which, in the case of DRAMs, also shortens the element's data holding time. Typically, a DRAM memory device relies on element capacity for data storage and receives a refresh command in a conventional standardized cycle, about every 100 milliseconds. However, with increasing element number and density, it is becoming more and more difficult to refresh all memory elements at least once within a refresh period. In addition, refresh operations consume device power, which is often limited. Finally, fabrication of these memory devices often requires complex silicon processing steps.
Recently, variable resistance memory elements have been investigated for suitability as semi-volatile and non-volatile random access memory elements. For example, U.S. Pat. No. 6,348,365, assigned to Micron Technology, Inc., and hereby incorporated by reference, discloses a programmable conductor random access memory device, including an insulating layer formed of a chalcogenide glass disposed between two electrodes. A conductive material, such as silver, is incorporated into the insulating layer. The resistance of the insulating layer can be programmed between high resistance and low resistance states by application of suitable voltages across the electrodes. These high and low resistance states can define two logic states.
Similar non-volatile molecular memory systems based on polymers are described in U.S. Patent Application Publication No. 2002/0163829 to Bulovic et al. (Bulovic) as well as in U.S. Patent Application Publication Nos. 2002/0163831 and 2003/0155602 to Krieger et al. (Krieger). The Bulovic and Krieger publications each describe memory devices in which a conductive polymer material is formed between two electrodes. As in the chalcogenide-based memory devices described above, the resistance of the polymer material can be switched from a high to a low resistance state in accordance with voltages applied across the electrodes, which can be used to define two logic states. Polymer-based memory systems show potential for use in both short-term and long-term data retention applications and are generally suitable for producing reliable and inexpensive memory devices. However, optimization of polymer-based memory systems can be difficult and time consuming, as forming various polymers selected for their particular electrical characteristics requires changing both reactants and reaction conditions.
Therefore, it is desirable to provide a polymer-based molecular memory device that is easier and less expensive to manufacture and allows flexibility in memory system design such that the polymer memory element can be quickly and efficiently optimized.